The present invention relates generally to gas turbine engines, and, more specifically, to turbine airfoil cooling.
In a gas turbine engine air is pressurized in a compressor and mixed with fuel in a combustor for generating hot combustion gases. Energy is extracted from the combustion gases in a high pressure turbine which powers the compressor, and further in a low pressure turbine which produces output power such as driving a fan in a typical turbofan aircraft engine application.
The high pressure turbine first receives the hottest combustion gases and is typically cooled for enhancing its durability and life. A high pressure turbine nozzle initially directs the hot combustion gases into the first row of high pressure turbine rotor blades extending radially outwardly from a supporting rotor disk.
The vanes and blades have suitable airfoil configurations for efficiently extracting energy from the combustion gases. The vane airfoils are hollow and suitably mounted at their radially outer and inner ends in corresponding stationary stator bands.
Each turbine blade includes a hollow airfoil and integral supporting dovetail which is mounted in a corresponding dovetail slot in the perimeter of the rotor disk for retention thereof. The row of rotor blades rotates during operation on the supporting disk for extracting energy from the combustion gases and driving the engine compressor.
Both the turbine nozzle vanes and turbine rotor blades require suitable cooling thereof during operation by providing thereto cooling air bled from the compressor. It is desirable to minimize the amount of cooling air bled from the compressor for maximizing efficiency and performance of the engine.
Accordingly, cooling configurations for the stator vanes and rotor blades have become quite sophisticated and esoteric over the many decades of continuing development thereof. Minor changes in cooling configurations of these components have significant affect on the cooling performance thereof, and in turn significantly affect efficiency and performance of the entire engine.
The airfoils of the vanes and blades may use similar cooling features, but suitably modified for the different configurations of the vanes and blades, and their different operation since the vanes are stationary, whereas the blades rotate during operation and are subject to considerable centrifugal forces.
The hollow airfoils of the vanes and blades typically have multiple radially or longitudinally extending cooling channels therein in one or more independent cooling circuits. The channels typically include small ribs or turbulators along the inner surface of the airfoils which trip the cooling air for enhancing heat transfer during the cooling process.
Typical cooling circuits include serpentine circuits wherein the cooling air is channeled successively through the serpentine legs for cooling the different portions of the airfoil prior to discharge therefrom.
The vanes and blades typically include various rows of film cooling holes through the pressure and suction sidewalls thereof which discharge the spent cooling air in corresponding films that provide additional thermal insulation or protection from the hot combustion gases which flow thereover during operation.
Yet another conventional cooling configuration includes separate impingement baffles or inserts disposed inside the nozzle vanes for impingement cooling the inner surface thereof. The baffles include a multitude of small impingement holes which typically direct the cooling air perpendicular to the inner surface of the vane for impingement cooling thereof. The spent impingement cooling air is then discharged from the vane through the various film cooling holes.
Impingement cooling of turbine rotor blades presents the additional problem of centrifugal force as the blades rotate during operation. Accordingly, turbine rotor blades typically do not use separate impingement baffles therein since they are impractical, and presently cannot meet the substantially long life requirements of modern gas turbine engines.
Instead, impingement cooling a turbine rotor blade is typically limited to small regions of the blade such as the leading edge or pressure or suction sidewalls thereof. Impingement cooling is introduced by incorporating a dedicated integral bridge or partition in the airfoil having one or more rows of impingement holes. Turbine rotor blades are typically manufactured by casting, which simultaneously forms the internal cooling circuits and the local impingement cooling channels.
The ability to introduce significant impingement cooling in a turbine rotor blade is a fundamental problem not shared by the nozzle stator vanes. And, impingement cooling results in a significant pressure drop of the cooling air, and therefore requires a corresponding driving pressure between the inside and outside of the airfoils during operation.
Since the pressure distribution of the combustion gases as they flow over the pressure and suction sides of the airfoils varies accordingly, the introduction of impingement cooling in turbine rotor blades must address the different discharge pressure outside the blades relative to a common inlet pressure of the cooling air first received through the blade dovetails in a typical manner.
Accordingly, it is desired to provide a turbine rotor blade having improved internal cooling therein.